Handling of FA brand icons is not great

[TimWolla]

The brand icons are currently special cased in IconFunctionTemplatePlugin, without any reusable class. This makes it harder to statically bake them into the template to avoid the I/O of loading the icon file for every request:

[pid 71525] openat(AT_FDCWD, "/var/www/html/icon/font-awesome/v6/brands/facebook.svg", O_RDONLY) = 8
[pid 71525] openat(AT_FDCWD, "/var/www/html/icon/font-awesome/v6/brands/twitter.svg", O_RDONLY) = 8
[pid 71525] openat(AT_FDCWD, "/var/www/html/icon/font-awesome/v6/brands/reddit.svg", O_RDONLY) = 8
[pid 71525] openat(AT_FDCWD, "/var/www/html/icon/font-awesome/v6/brands/linkedin-in.svg", O_RDONLY) = 8
[pid 71525] openat(AT_FDCWD, "/var/www/html/icon/font-awesome/v6/brands/pinterest.svg", O_RDONLY) = 8